1.Field of the Invention
The present invention relates to a method for detecting a defect of a transparent body and a method for producing a transparent body. In particular, the present invention relates to a method for detecting a defect of a transparent body, which is preferably used to detect the defect such as the bubble, the foreign matter, and the surface scratch existing, for example, in an acrylic plate, as well as the bend, the waviness, and the warpage of the acrylic plate itself. The present invention also relates to a method for producing a transparent body, in which an obtained result of detection of the defect is sent in a feedback manner to a production line for the transparent body to make changes to the production of the transparent body.
2. Description of the Related Art
Those hitherto known as the display include, for example, cathode ray tubes (CRT) and liquid crystal display apparatuses.
The material, which is often used for an optical guide plate of such a display, is glass or a transparent material, because it is necessary to totally reflect, within the optical guide plate, the light introduced from a light source.
When the optical guide plate is composed of glass, the following problems occur. That is, the weight of the optical guide plate is large when the display surface has a large size. Further, the price of glass is expensive.
In order to solve the problems as described above, an acrylic plate is also used as a material for the optical guide plate.
However, as shown in FIG. 15, when an optical guide plate is produced by using an acrylic plate 200, a scratch 202 tends to be formed on its surface. Further, for example, a bubble 204 and a foreign matter 206 sometimes appear in the acrylic plate 200. Furthermore, the flatness is occasionally lost due to any bend or waviness of the acrylic plate 200 itself.
Because of the circumstances as described above, an inspection is made whether any defect as described above is present or absent by means of visual observation effected by human eyes, during the inspection step for the products of the acrylic plates 200. However, such a process is not necessarily efficient.
On the other hand, a technique for inspecting defect of a transparent body has been disclosed in which a light beam originating from a light source is radiated onto a plate member to inspect the plate member for its defect or the like (see, for example, Japanese Laid-Open Patent Publication No. 58-158920). An apparatus for inspecting the defect described in this document is operated as follows for the foreign matter and the scratch on the surface. That is, the light beam originating from the light source regulated by a slit in the radiation direction is radiated in parallel to a transparent body such as a glass mask for photolithography (in parallel to a principal surface of the transparent body), and scattered light concerning the foreign matter and the scratch on the surface is received by using a light-receiving unit so that the inspection is made for the presence or absence of the foreign matter and the surface scratch directed to the mask aperture.
However, in the case of the conventional apparatus for inspecting the defect, it is required to radiate the parallel light beam onto the plate member, and hence it is necessary to newly provide any optical system for producing the parallel light beam from the light supplied, for example, from a point light source. Therefore, it is feared that the system may be complicated. Further, a problem arises in that the positional control to radiate the parallel light beam onto the entire transparent body is difficult to implement.
When the parallel light beam is used, the following inconvenience arises. That is, it is impossible to detect any foreign matter or the like adhering to the surface of the plate member if the slit is located on the side surface of the plate member. When the surface of the foreign matter existing in the plate member is a mirror surface, the reflected light of the parallel light beam is not transmitted to the side of the principal surface depending on the position and the angle thereof. The reflected light is transmitted toward the side surface of the transparent body, and it cannot be detected by the light-receiving unit which is installed over the principal surface of the transparent body.
As understood from the principle of detection, the conventional technique described above makes it possible to detect a surface scratch and the foreign matter existing at the inside of the transparent body. However, the conventional technique fails to detect, for example, the bend and the waviness of the transparent body itself.
Further, in the conventional technique, the parallel light beam is introduced via a slit into a measurement space in which the transparent body is installed. For this reason, in order to perform the detection of the bubble, the foreign matter and the like existing in the transparent body and the detection of the scratch existing on the surface, it is necessary to move the slit in the thickness direction of the transparent body. Therefore, it is impossible to simultaneously perform the two detection processes described above. When the transparent body has a large thickness, a new problem arises in that the detection requires a lot of time and labor.